Different types of hydraulic dissipators or dampers are known, also commonly called viscous dissipators.
In particular, a type of viscous dissipator is a cylinder-piston mechanism capable of damping the movement transferred to it by means of viscous friction. The viscous force with which the dissipator reacts to the action is proportional to the speed and has an opposite direction to the input force. Such device is often used in combination with an elastic element, in particular a spring.
More in particular, the piston that slides in the cylinder defines a first and a second chamber separate from each other, in such a way that during the stroke of the piston there is a leakage of the viscous fluid between the first and the second chamber.
In order to obtain the leakage that determines the rate of dissipation of the damper, dedicated fixed passages are provided, or valves, on the piston. These are represented, for example, by holes with minimum diameter that damp the leakage of the fluid from a chamber to the other and then determine the dissipation.
A further example provides instead the presence of valve elements suitable to pass from a more open configuration to a more closed configuration changing the damping force.
However the above described hydraulic dampers are not effective if they have to dissipate or damp a plurality of actions temporally close to each other. It emerged, in fact, that the leakage of the viscous fluid through the valve means affects both the forward stroke both the back stroke of the piston. Therefore, the piston takes substantially the same time to go back to a starting position as the forward stroke.
In addition, such types of dissipators are not capable to have a horizontal position, or inclined, since in this case the valve element cannot return spontaneously to the starting position.
It is desirable, for example in the nautical field, to have dampers suitable for damping the actions caused by waves on a watercraft. To this end, spring damping devices are known, as described in WO9006453A1, or with resilient rubber elements, as described in WO03106251A1, arranged between two portions of a mooring, suitable for damping movements of the watercraft, caused by waves, when the watercraft is moored, for example, in a harbour.
On the other hand, the use of other damping systems different from those above cited would not be suitable to this object, since the viscous dampers of known type are not capable of dissipating actions that are temporally close to each other, like a movement caused by waves. In fact, according to the principle of operation of the above described shock absorbers, a movement of the watercraft away from the point of mooring, causes mooring ropes to be pulled abruptly and damped by the shock absorber, in order not to cause back stroke to the watercraft and negatively affecting the comfort of the passengers. However, the cylinder-piston mechanism if subject to pulling forces close to each other, owing for example to a succession of waves that displace the watercraft, once done a forward stroke and arrived to the second dead point of the damper, after damping a first wave, requires a time substantially equal for carrying out the back stroke and returning to the first dead point of the damper.
Therefore, in case of waves temporally close to each other the shock absorber loses progressively its function damping correctly the waves only if they are regularly paced. More in particular, the piston that slides in the cylinder, after having damped the first wave, starts to carry out the back stroke, and if a second wave arrives close to the first, the piston will be located in an intermediate position between the second dead point and the first dead point of the damper, thus reducing remarkably its damping effectiveness. In particular, for a succession of waves very close the shock absorber it can lose almost completely its damping effectiveness.
DE102008042822A1 describes an hydraulic damper of viscous type, with adjustable viscous resistance. The shock absorber also comprises plugs on the head of the piston in the form of leaf springs, which are foldably connected to a fastening point. Such plugs, if they were subject to high deformation elevate, would have the drawback of a quick wear and then break. In any case, such an hydraulic damper would not solve the problem of a quick back stroke, as it would be desirable in case of shock absorbers for watercrafts.
In case of shock absorbers for damping the effect of the waves on watercrafts moored in a harbour, as well as for other applications, it would be also desirable to exploit the energy accumulated by the shock absorbers for generating electric energy, for example for recharging batteries on wharfs or on watercrafts, or for any other use.